It is widely accepted that oil recovery during waterflooding can be improved by modifying the composition of the injected brine in a process called controlled salinity waterflooding (CSW). However, despite a plethora of studies and data, the underlying mechanisms causing improved recovery remain poorly understood. Moreover, there is no method to determine the optimum injection brine composition for a given crude-oil-brine-rock system. Many studies have reported the successful application of CSW . However, many others (and more unpublished) observed no benefit, and the available data are often inconsistent and contradictory. This presentation will give a brief overview of the available data, and the pore- to mineral-surface-scale mechanisms that have been proposed to explain improved recovery. New data will be presented that suggests improved oil recovery during CSW is strongly correlated to changes in zeta potential at both the mineral-water and oil-water interfaces. Improved recovery in the experiments reported here occurred only when the change in brine composition induced a repulsive electrostatic force between the oil-brine and mineral-brine interfaces. The results suggest that the polarity of the zeta potential at both interfaces must be determined when designing the optimum CSW composition. A key challenge for any model of CSW is to explain why improved recovery is not always observed. Here we suggest that failures using the conventional (dilution) approach to CSW may have been caused by a positively charged oil-water interface that had not been identified.